The present invention relates to facsimile control systems using a multi-microcomputer system. More particularly, the invention relates to a facsimile control system using a multi-microcomputer system in which a plurality of microcomputers are connected in a hierarchical arrangement so that various controls and system supervision can be carried out with a high efficiency.
Due to the recent rapid progress in semiconductor manufacturing techniques, one-chip microcomputers incorporating a central processing unit (CPU) having calculation and decision functions, a random access memory (RAM) for storing data, a read-only memory (ROM) for storing processing programs, and an input/output control unit (I/O) have become available at a relatively low price. An example of such a one-chip microcomputer is as shown in FIG. 1.
These one-chip microcomputers have been used in facsimile systems to control and detect various functions and data operations.
For instance in the case of telephony facsimile which is most extensively employed, among the various control functions, the following can be suitably performed by a microcomputer:
(1) Line control
A so-called "connection control" for calling and incoming signals.
(2) Transmission control
Controlling of procedures for mutually transmitting and receiving control data and video data between the calling station and the called station after the telephone line has been engaged and connected.
(3) Mechanism control
Controlling of the operations of the various operations in the facsimile system such as controlling the start and stop of sub-scanning.
(4) Man machine interface control
Sensing of operating switches and controlling of various displays.
An example of microcomputer and peripheral circuit employable in such a control system is shown in FIG. 2. In FIG. 2, a microcomputer M includes on a single chip a program ROM, a work area registering RAM, input ports A and B, output ports C and D, a programmable timer PGT, a clock input terminal CL, a reset input terminal RS and a halt signal input terminal HT. A multiplexer MP is provided to increase the number of input/output ports with the multiplexer MP being controlled by the output CS of the microcomputer M.
In order to control facsimile transmission and reception with such a microcomputer, it is necessary to provide a number of input/output sections. However, since the number of input/output terminals is limited for a one-chip microcomputer, the shortage of input/output terminals is complemented by adding a latch circuit or a multiplexer externally as shown in FIG. 2. The cost of adding such a latch circuit or a multiplexer is however high, sometimes higher than the cost of the microcomputer. Furthermore, since it is necessary to simultaneously perform the above-described various control and check functions, sometimes the employment of only one microcomputer is insufficient because the microcomputer would be overloaded and the read-only memory built into the microcomputer may be inadequate in capacity because of long program steps. In order to overcome this difficulty, a technique has been proposed in the art in which data is processed by a multimicrocomputer system using a plurality of microcomputers.
In general, in a conventional multi-computer system, in order to mutually transmit and receive data between the computers, the computers are connected with a common data bus line and a control line and data exchange is carried out between the computers after signal confirmation in such a manner that, when one of the computers transmits a data transfer signal to the other computer, the other computer outputs an acknowledge signal upon receipt of the data transfer signal. However, it is impossible for a one-chip microcomputer to employ this technique because the data bus line and the control line do not extend outside the chip.